Electrostatic pressure ink jet systems are generally of one of two types. An example of one type is represented by Sweet U.S. Pat. No. 3,596,275 wherein pressurized electrically conductive fluid is ejected from a single nozzle as a fluid filament and perturbated for breaking into a stream of uniform drops. As each drop breaks off from its fluid filament, it may be selectively and variably charged by a charge field from a charge electrode. Th drops then pass through an electrostatic deflection field and are deflected from the normal path a distance which is related to the magnitude of the charges carried by the droplets. The droplets may then impact a recording medium at a location determined by the deflection distance. The other type of system is represented by Sweet et al. U.S. Pat. No. 3,373,437 wherein the pressurized electrically conductive fluid is ejected from a plurality of orifices and broken into plural streams of uniform drops. Again, as each drop breaks off from its fluid filament, it may be selectively charged by an associated charge electrode. Rather than variably charging the drops, this type of system operates binarily, giving a drop either a predetermined charge or leaving it in an uncharged condition. The drops then pass through an electrostatic deflection field, the charged drops being deflected thereby to a drop catcher or gutter, while the uncharged drops are undeflected and continue past the deflection field to impact a recording medium for printing.
The charge electrodes previously used comprise an electrically conductive material partially or wholly surrounding the corresponding ink jet stream and extending uniformly along the stream a distance of several drops. The reason for this is that the drop is charged in accordance with the field established by the charge electrode at the instant the drop breaks off from the filament, but the precise breakoff point can vary axially along the stream depending upon such factors as fluid viscosity and pressure. The field must be uniform along this axial distance so that the drops are properly charged without regard to the precise point of breakoff. For example, the charge electrode shown in Sweet U.S. Pat. No. 3,596,275 comprises an elongate ring or tube. In Sweet et al. U.S. Pat. No. 3,373,437, the charge electrode comprises a U-shaped channel.
Insertion of the ring or the channel electrodes into a supporting structure and then connecting the charge electrodes to a charging signal source is difficult and costly, especially in the multi-jet binary ink jet systems such as the Sweet et al. patent above.
Various attempts have been made to reduce difficulty and cost of forming tubular charge electrodes. One example is Beam et al. U.S. Pat. No. 3,586,907 which illustrates a charge ring plate provided with a series of holes and having a discrete application of an electrically conductive coating material surrounding each hole in circular outline and covering the wall thereof to form a charge ring. Also applied to the plate are a series of discrete lines of the electrically conductive coating material to form leads extending from each charge ring to a connector. This technique, also known as "through-hole plating" involves plating in several dimensions and exposes the charge electrode to contact with the mist or splash of the electrically conductive ink, which could result in electrical shorting.
Another example is represented by Robertson U.S. Pat. No. 3,604,980 which is formed from a dielectric support plate having a planar side surface placed alongside a plurality of streams and provided with strips of conductive material thereon adjacent each stream and connected to plated leads connected to the charging signal sources. Again, this type of electrode involves plating in several dimensions. This type of charge electrode however, is adjacent only one side of each stream and does not surround the stream, resulting in a poor charge field. Direct contact with the ink mist or splash may be avoided by applicaion of a common insulating layer, but such a coating may be subject to pin holes or defects, resulting in possible electrical shorting due to the ink.
Still another example is Culp U.S. Pat. No. 3,618,858 which discloses a dielectric bar with a series of U-shaped notches, each notch having a coating of an electrically conductive material applied to the wall to form a U-shaped channel and connected by a printed circuit lead extending from each notch to a terminal. Once again, the plating must be accomplished in several dimensions and exposes the electrodes possible shorting from the electrically conductive ink.